\documentclass{standalone}

\begin{document}

\def\foo{\relax}
\ifx\foo\relax
  \typeout{foo is relax}
\else
  \typeout{foo is NOT relax}
\fi

\let\foo=\relax
\ifx\foo\relax
  \typeout{foo is relax}
\else
  \typeout{foo is NOT relax}
\fi

\end{document}